In the production of alumina from bauxite, a variety of crystallisation processes occur. Desilication product, product hydrate, gibbsite scale, tricalcium aluminate, calcium carbonate, and sodium oxalate are some of the materials which crystallise from process liquors supersaturated at various stages in the Bayer process. In many cases, the crystalline material is detrimental to the refinery operation.
The control of Bayer process parameters has been the only practical method available for altering process crystallisation. Unfortunately, the process control techniques employed to reduce crystallisation problems often reduce process efficiency.
Specialty chemical companies are now marketing "crystal modifiers" to the alumina industry for controlling process crystallisation. These reagents have been "designed" to improve crystal quality and to permit the operation of a unit process under conditions that would ordinarily cause crystallisation problems. However, the basic mechanisms by which process chemicals could be used to control crystallisation in alumina processing are not well understood.
The purpose of this paper is to briefly review nucleation and growth theory and to examine some of the ways reagents might be used to control process crystallisation in alumina refining.